Heart rhythm disorders claim 300-400,000 lives of Americans annually. Existing therapies are primarily based on pharmacological, surgical, ablative, and implantable device approaches. One of the major impediments in diagnostics is the inability to record action potentials (AP) in human patients noninvasively. The only available technique is an invasive intracardiac MAP catheter, which provides high fidelity recording from a single site and thus far has not been adopted for high resolution mapping. Alternatively, optical mapping technology permits recordings of tens of thousands of APs from the epicardium or endocardium, yet it has been used solely for in vitro studies and has not yet been translated to the clinical electrophysiology (EP) laboratory. Current clinical EP studies rely on electrograms, which are the spatially integrated electrical signatures of action potentials produced by numerous cells. Hence, they are not easily interpretable in complex conduction and repolarization cases. The advent of electrocardiographic imaging (ECGi) based on inverse problem solution, has provided an exciting opportunity to map epicardial electrograms noninvasively starting from the body surface electrogram mapping and whole thorax imaging using CT or MRI. However, this method provides only electrograms and not the action potentials. Moreover, body surface potential mapping modalities thus far cannot recreate endocardial electrograms, which requires invasive transvenous balloon or basket catheter mapping. We have developed a novel AP ECGi methodology which aims to noninvasively reconstruct action potentials based on body surface electrogram measurement and inverse problem solution. However, the critical issue is how to validate these methodologies, because there is no high resolution mapping technique available for direct mapping of action potentials in vivo. In this project we will develop an ex vivo validation platform against which all existing ECG methodologies and our new AP ECGi can be directly compared, using simultaneous mapping of electrograms and optical action potentials. We will create an ex vivo 3D model of the human thorax surrounding Langendorff-perfused human or rabbit hearts. The system will simulate a living heart in a human thorax and allow the simultaneous acquisition of (i) 240 thorax surface electrograms and (ii) panoramic mapping of optical action potentials from the epicardium using voltage-sensitive dyes. Our novel method will allow significant advancement of the existing noninvasive clinical cardiac electrophysiology armamentarium by validation of AP ECGi. Moreover, we will make these data sets available to other research groups and companies that are interested in validation of their own competing ECGi methods.